The long-term goal of this project is to elucidate the mechanisms of Salmonella pathogenesis that lead to serious, disseminated infections. Salmonella strains are an increasingly common cause of food-borne illness in the United States and throughout the world. The estimated annual cost of Salmonella disease in the U.S. alone is over $1.4 billion. A large body of experimental work has shown that Salmonella infections require CD4+ T cells for resolution, and immunosuppressed individuals such as those with HIV infection or cancer are at high risk of serious Salmonella disease. Certain non-typhoid strains of Salmonella carry virulence plasmids with the spv gene locus that encodes factors promoting more severe Salmonella infections. This application proposes a novel hypothesis that the spv locus is the principal virulence factor that allows Salmonella to overcome innate immunity and that explains the requirement for CD4+ cells for effective, acquired immunity. Salmonella strains are able to grow inside host defense cells such as macrophages and eventually induce apoptosis of infected cells by a process dependent on the Spv proteins. The SpvB protein blocks actin polymerization, while SpvC inhibits MAP kinase signaling. This project will test specific hypotheses based on an overall model for the role of the Spv proteins during Salmonella infection. SpvB is proposed to inhibit macrophage function through loss of the actin cytoskeleton, while SpvC is postulated to reduce innate immune activation. SpvB and SpvC both act to promote apoptosis of infected cells and enhance cell to cell spread of Salmonella infection. The specific aims of the proposal will test these hypotheses through 1) Characterization of the virulence mechanisms specified by SpvC and relationships to the action of SpvB, and 2) Characterization of the mechanisms used by SpvB and SpvC to overcome innate immunity, and 3) Determine the role of CD4+ T cells in the immunity to infection with spv+ Salmonella strains.